Methods for variable multi-pass servowriting and self-servowriting

ABSTRACT

The amount of position error written into a servo burst pattern can be reduced by using additional media revolutions to write the pattern. Where the edges of two servo bursts are used to define a position on the media, trimming the first burst and writing the second burst on separate revolutions will result in a different amount of position error being written into each burst. The end result will be a reduction in the overall error in position information. In order to further reduce the position error given by a burst pair, each burst also can be trimmed and/or written in multiple passes. This description is not intended to be a complete description of, or limit the scope of, the invention. Other features, aspects, and objects of the invention can be obtained from a review of the specification, the figures, and the claims.

CLAIM OF PRIORITY

This application is a continuation application of U.S. patentapplication Ser. No. 10/630,522, filed Jul. 30, 2003, now abandondedwhich claims benefit from U.S. Provisional Patent Application No.60/436,740, filed Dec. 27, 2002 and U.S. Provisional Patent ApplicationNo. 60/436,673, filed Dec. 27, 2002, all of which are incorporatedherein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following applications are cross-referenced and incorporated hereinby reference:

U.S. Provisional Patent Application No. 60/436,712 entitled “Systems forSelf-Servowriting Using Write-Current Variation,” by Richard M. Ehrlich,filed Dec. 27, 2002.

U.S. Provisional Patent Application No. 60/436,703 entitled “Methods forSelf-Servowriting Using Write-Current Variation,” by Richard M. Ehrlich,filed Dec. 27, 2002.

U.S. patent application Ser. No. 10/420,076 entitled “Systems forSelf-Servowriting Using Write-Current Variation,” by Richard M. Ehrlich,filed Apr. 22, 2003.

U.S. patent application Ser. No. 10/420,498 entitled “Methods forSelf-Servowriting Using Write-Current Variation,” by Richard M. Ehrlich,filed Apr. 22, 2003.

U.S. patent application Ser. No. 10/818,473, entitled “Systems forSelf-Servowriting Using Write-Current Variation,” by Richard M. Ehrlich,filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,174, entitled “Methods forSelf-Servowriting Using Write-Current Variation,” by Richard M. Ehrlich,filed Apr. 5, 2004.

U.S. Provisional Patent Application No. 60/436,709 entitled “Systems forMulti-Pass Self-Servowriting,” by Richard M. Ehrlich, filed Dec. 27,2002.

U.S. Provisional Patent Application No. 60/436,743 entitled “Methods forMulti-Pass Self-Servowriting,” by Richard M. Ehrlich, filed Dec. 27,2002.

U.S. patent application Ser. No. 10/420,452 entitled “Systems forSelf-Servowriting With Multiple Passes Per Servowriting Step,” byRichard M. Ehrlich, filed Apr. 22, 2003.

U.S. patent application Ser. No. 10/420,127 entitled “Methods forSelf-Servowriting With Multiple Passes Per Servowriting Step,” byRichard M. Ehrlich, filed Apr. 22, 2003.

U.S. patent application Ser. No. 10/818,181, entitled “Systems forSelf-Servowriting With Multiple Passes Per Servowriting Step,” byRichard M. Ehrlick, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,185, entitled “Methods forSelf-Servowriting With Multiple Passes Per Servowriting Step,” byRichard M. Ehrlich, filed Apr. 5, 2004.

U.S. Provisional Patent Application No. 60/436,744 entitled “SystemsUsing Extended Servo Patterns with Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Dec. 27, 2002.

U.S. Provisional Patent Application No. 60/436,704 entitled “MethodsUsing Extended Servo Patterns with Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Dec. 27, 2002.

U.S. patent application Ser. No. 10/630,523 entitled “Systems UsingExtended Servo Patterns with Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Jul. 30, 2003.

U.S. patent application Ser. No. 10/630,528 entitled “Methods UsingExtended Servo Patterns with Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Jul. 30, 2003.

U.S. patent application Ser. No. 10/630,521 entitled “Systems UsingExtended Servo Patterns with Variable Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Jul. 30, 2003.

U.S. patent application Ser. No. 10/630,524 entitled “Methods UsingExtended Servo Patterns with Variable Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Jul. 30, 2003.

U.S. patent application Ser. No. 10/818,180, entitled “Systems UsingExtended Servo Patterns with Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,175, entitled “Methods UsingExtended Servo Patterns with Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,445, entitled “Systems UsingExtended Servo Patterns with Variable Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,179, entitled “Methods UsingExtended Servo Patterns with Variable Multi-Pass Servowriting andSelf-Servowriting,” by Richard M. Ehrlich, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/622,233 entitled “Systems forSelective Multi-Pass Servowriting and Self-Servowriting,” by Richard M.Ehrlich, filed Jul. 18, 2003.

U.S. patent application Ser. No. 10/622,215 entitled “Methods forSelective Multi-Pass Servowriting and Self-Servowriting,” by Richard M.Ehrlich, filed Jul. 18, 2003.

U.S. patent application Ser. No. 10/630,219 entitled “Systems forVariable Multi-Pass Servowriting and Self-Servowriting,” by Richard M.Ehrlich, filed Jul. 30, 2003.

U.S. patent application Ser. No. 10/818,450, entitled “Systems forSelective Multi-Pass Servowriting and Self-Servowriting,” by Richard M.Ehrlich, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,472, entitled “Methods forSelective Multi-Pass Servowriting and Self-Servowriting,” by Richard M.Ehrlich, filed Apr. 5, 2004.

U.S. patent application Ser. No. 10/818,177, entitled “Methods forVariable Multi-Pass Servowriting and Self-Servowriting,” by Richard M.Ehrlich, filed Apr. 5, 2004.

FIELD OF THE INVENTION

The present invention relates to servowriting processes and devices.

BACKGROUND

Advances in data storage technology have provided for ever-increasingstorage capability in devices such as DVD-ROMs, optical drives, and diskdrives. In hard disk drives, for example, the width of a written datatrack has decreased due in part to advances in read/write headtechnology, as well as in reading, writing, and positioningtechnologies. More narrow data tracks result in higher density drives,which is good for the consumer but creates new challenges for drivemanufacturers. As the density of the data increases, the tolerance forerror in the position of a drive component such as a read/write headdecreases. As the position of such a head relative to a data trackbecomes more important, so too does the placement of information, suchas servo data, that is used to determine the position of a head relativeto a data track.

BRIEF SUMMARY

Systems and methods in accordance with the present invention takeadvantage of multiple passes in servowriting and self-servowritingapplications. These additional passes allow patterns such as data burstpairs to be written and/or trimmed on separate passes. The additionalpasses reduce the written runout, as the average misplacement decreaseswhen the number of passes increases. Each burst in a servo pattern canalso be written and/or trimmed in multiple passes. Additional passes canalso be used only for selected tracks or servo bursts on a disk.

Other features, aspects, and objects of the invention can be obtainedfrom a review of the specification, the figures, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing components of a disc drive that can be usedin accordance with embodiments of the present invention.

FIG. 2 is a diagram showing an example of a data and servo format for adisk in the drive of FIG. 1.

FIG. 3 is a diagram showing servo information that can be written to thetracks shown in FIG. 2.

FIGS. 4( a)–(f) are diagrams of a servo-burst pattern being written overa progression of servowriting steps.

FIGS. 5 and 6 are diagrams of a servo-burst pattern being written over aprogression of servowriting steps, wherein there is a head misplacementon the second step.

FIG. 7 is a diagram of a servo-burst pattern being written over aprogression of servowriting steps using multiple passes in accordancewith one embodiment of the present invention.

FIG. 8 is a diagram of a servo-burst pattern being written over aprogression of servowriting steps using multiple passes in accordancewith a second embodiment of the present invention.

FIG. 9 is flowchart for a method that can be used with to write thepatterns of FIGS. 7 and 8.

FIG. 10 is a diagram of a servo-burst pattern wherein a portion of thepattern is written over a progression of servowriting steps usingmultiple passes in accordance with a third embodiment of the presentinvention.

FIG. 11 is a diagram of a servo-burst pattern that can be used inaccordance with embodiments of the present invention.

FIG. 12 is a diagram of another servo-burst pattern that can be used inaccordance with embodiments of the present invention.

FIG. 13 is a diagram of another servo-burst pattern that can be used inaccordance with embodiments of the present invention.

DETAILED DESCRIPTION

Systems and methods in accordance with one embodiment of the presentinvention can be used when servowriting, or self-servowriting, arotatable storage medium in a data storage device, such as a hard diskdrive. For example, a typical disk drive 100, as shown in FIG. 1,includes at least one magnetic disk 102 capable of storing informationon at least one of the surfaces of the disk. A closed-loop servo systemcan be used to move an actuator arm 106 and data head 104 over thesurface of the disk, such that information can be written to, and readfrom, the surface of the disk. The closed-loop servo system can contain,for example, a voice coil motor driver 108 to drive current through avoice coil motor (not shown) in order to drive the actuator arm, aspindle motor driver 112 to drive current through a spindle motor (notshown) in order to rotate the disk(s), a microprocessor 120 to controlthe motors, and a disk controller 118 to transfer information betweenthe microprocessor, buffer, read channel, and a host 122. A host can beany device, apparatus, or system capable of utilizing the data storagedevice, such as a personal computer or Web server. The drive can containat least one processor, or microprocessor 120, that can processinformation for the disk controller 118, read/write channel 114, VCMdriver 108, or spindle driver 112. The microprocessor can also include aservo controller, which can exist as an algorithm resident in themicroprocessor 120. The disk controller 118, which can store informationin buffer memory 110 resident in the drive, can also provide user datato a read/write channel 114, which can send data signals to a currentamplifier or preamp 116 to be written to the disk(s) 102, and can sendservo and/or user data signals back to the disk controller 118.

The information stored on such a disk can be written in concentrictracks, extending from near the inner diameter of the disk to near theouter diameter of the disk 200, as shown in the example disk of FIG. 2.In an embedded servo-type system, servo information can be written inservo wedges 202, and can be recorded on tracks 204 that can alsocontain data 206. In a system where the actuator arm rotates about apivot point such as a bearing, the servo wedges may not extend linearlyfrom the inner diameter (ID) of the disk to the outer diameter (OD), butmay be curved slightly in order to adjust for the trajectory of the headas it sweeps across the disk.

The servo information often includes bursts of transitions called “servobursts.” The servo information can be positioned regularly about eachtrack, such that when a data head reads the servo information, arelative position of the head can be determined that can be used by aservo processor to adjust the position of the head relative to thetrack. For each servo wedge, this relative position can be determined inone example as a function of the target location, a track number readfrom the servo wedge, and the amplitudes or phases of the bursts, or asubset of those bursts. The position of a head or element, such as aread/write head or element, relative to the center of a target track,will be referred to herein as a position-error signal (PES).

For example, a centerline 300 for a given data track can be “defined”relative to a series of bursts, burst edges, or burst boundaries, suchas a burst boundary defined by the lower edge of A-burst 302 and theupper edge of B-burst 304 in FIG. 3. The centerline can also be definedby, or offset relative to, any function or combination of bursts orburst patterns. This can include, for example, a location at which thePES value is a maximum, a minimum, or a fraction or percentage thereof.Any location relative to a function of the bursts can be selected todefine track position. For example, if a read head evenly straddles anA-burst and a B-burst, or portions thereof, then servo demodulationcircuitry in communication with the head can produce equal amplitudemeasurements for the two bursts, as the portion of the signal comingfrom the A-burst above the centerline is approximately equal inamplitude to the portion coming from the B-burst below the centerline.The resulting computed PES can be zero if the radial location defined bythe A-burst/B-burst (A/B) combination, or A/B boundary, is the center ofa data track, or a track centerline. In such an embodiment, the radiallocation at which the PES value is zero can be referred to as anull-point. Null-points can be used in each servo wedge to define arelative position of a track. If the head is too far towards the outerdiameter of the disk, or above the centerline in FIG. 3, then there willbe a greater contribution from the A-burst that results in a more“negative” PES. Using the negative PES, the servo controller coulddirect the voice coil motor to move the head toward the inner diameterof the disk and closer to its desired position relative to thecenterline. This can be done for each set of burst edges defining theshape of that track about the disk.

The PES scheme described above is one of many possible schemes forcombining the track number read from a servo wedge and the phases oramplitudes of the servo bursts. Many other schemes are possible that canbenefit from embodiments in accordance with the present invention.

A problem that exists in the reading and writing of servo patternsinvolves the misplacement, or offset, of a read/write head with respectto the ideal and/or actual position of a track. It is impossible toperfectly position a head with respect to a track for each rotation of adisk, as there is almost always a noticeable offset between the desiredposition and the actual position of the head with respect to the disk.This can cause problems when writing servo patterns, as each portion ofthe pattern can be slightly misplaced. This can lead to what is referredto as written-in runout. Written-in runout can be thought of as theoffset between the “actual” centerline, or desired radial center, of atrack and the centerline that would be determined by a head reading thewritten servo pattern. Written-in runout can lead to servo performanceproblems, wasted space on a disk and, in a worst case, unrecoverable orirreparably damaged data.

Systems and methods in accordance with embodiments of the presentinvention overcome deficiencies in prior art servowriting andself-servowriting systems by taking advantage of additional passes whenwriting servo information. The use of additional passes for the writingand/or trimming of servo burst patterns, for example, can provide for alow written-in runout in a servo pattern, but at the cost of sometime-penalties in the servowriting and/or self-servowriting operations.The additional passes can achieve this reduced written-in runout byeffectively making the written-in runout be the average of multipleservowriting passes. The time penalty due to the additional passes issmall in self-servowriting processes, since a drive typically alreadyspends many revolutions at each servowriting position in determinationof the written-in runout of the reference pattern. One or two extrarevolutions will only increase the self-servowriting time by a smallfraction, such as on the order of about 16%–32%. Used with a standardservowriting process, each additional pass can add on the order of 75%.

FIGS. 4( a)–4(f) depict the progression of several steps of an exemplaryservowriting process. The pattern shown in these figures is commonlyreferred to in the trade as a 3-pass-per-track, trimmed-burst pattern,for reasons described below. However, it is to be understood that forthis specification the appropriate term is “3-step-per-track” or“3-servowriting-step-per-track”. That is to say that each servowritingstep can include one or multiple passes and each track is defined by oneor multiple servowriting steps. Each figure depicts a small portion ofthe surface of a disk. This portion can contain several servo tracks,extending radially on the disk and vertically in the figures, and cancover the space of a single servo wedge, circumferentially on the diskand horizontally in the figures. A typical drive can have tens ofthousands of servo tracks, and over one hundred wedges per revolution.In the figures, the patterned areas indicate portions of the surface ofthe disk that have been magnetized in one direction. Areas withoutpatterning have been magnetized in another direction, typically in adirection opposite to that of the patterned areas. For drives which uselongitudinal recording, the first direction will be substantially in thecircumferential direction, and the second direction will be opposite tothe first. For drives which use perpendicular recording (also referredto as “vertical recording”), the two directions are perpendicular to theplane of the disk. These simplified figures do not show effects ofside-writing of the write element, which can produce non-longitudinalmagnetization and erase bands. Such effects are not of primaryimportance to the discussion herein.

In FIG. 4( a), the result of a single servowriting step is shown. Fromthat step, the servowriting head (passing from left to right in thefigure) has written an exemplary servo pattern containing a preamble,followed by a servo-address mark (SAM), followed by an INDEX-bit, andthen a track number, as is known in the art. Other information can bewritten to the servo pattern in addition to, or in place of, theinformation shown in FIG. 4( a). An INDEX-bit, for example, is one pieceof information that can be used to give the servo an indication of whichwedge is wedge-number zero, useful for determining circumferentialposition. The track number, which can be a graycoded track-number, canlater be used by the servo to determine the coarse radial position ofthe read/write (R/W) head. Following the track number, the head writesone of four servo bursts, in this case what will be referred to as aC-burst, which can later be used by a servo to determine the fine(fractional track) radial position of a R/W head. The number of servobursts used can vary with servo pattern. The burst that is written canbe, for example, the one that is in-line with the digital information.The width of the written track can be determined by the magneticwrite-width of the write element of the servowriting head.

FIG. 4( b) shows the result of a second step of the servowritingprocess. All that has been added in the second step is an additionalburst, in this case referred to as an A-burst. The A-burst is displacedlongitudinally from both the digital information and the C-burst, toprevent any overlap in the longitudinal direction. The A-burst is alsodisplaced by approximately one-half of a servo-track in the radialdirection.

FIG. 4( c) shows the magnetization pattern after three steps of theservowriting process. The new portion of the pattern has been writtenwith the servowriting head displaced another half servo track radially,for a total displacement of one servo-track, or two-thirds of adata-track, from the position of the head during the first pass. Newdigital information has been written, including the same preamble, SAM,and INDEX-bit, as well as a new track number. A D-burst was added duringthe third servowriting step, and the C-burst was “trimmed.” The C-burstwas trimmed by “erasing” the portion of the C-burst under theservowriting head as the head passed over the burst on the thirdservowriting step. As long as the servowriting head is at leasttwo-thirds of a data-track in radial extent, the digital informationwill extend across the entire radial extent of the servowritten pattern.This trimming of the C-burst and writing of the D-burst created a commonedge position or “boundary” between the two bursts.

In FIG. 4( d), a B-burst has been added and the A-burst trimmed in thefourth step of the servowriting process. At a point in time after theservowriting is complete, such as during normal operation of the diskdrive, the upper edge of the B-burst and the lower edge of the A-burstcan be used by the servo, along with the graycoded track-number whoseradial center is aligned with the burst edges, to determine the R/W headposition when it is in the vicinity of the center of that servo track.If a reader evenly straddles the A-burst and the B-burst, the amplitudeof the signals from the two bursts will be approximately equal and thefractional Position-Error Signal (PES) derived from those bursts will beabout 0. If the reader is off-center, the PES will be non-zero,indicating that the amplitude read from the A-burst is either greaterthan or less than the amplitude read from the B-burst, as indicated bythe polarity of the PES signal. The position of the head can then beadjusted accordingly. For instance, negative PES might indicate that theamplitude read from the A-burst is greater than the amplitude read fromthe B-burst. In this case, the head is too far above the center position(using the portion of the pattern in the figure) and should be movedradially downward/inward until the PES signal is approximately 0. Itshould be noted that for other portions of the pattern a B-burst couldbe above an A-burst, resulting in a negative amplitude contributioncoming from the A-burst. Other burst-demodulation schemes have beenproposed which determine the PES as a function of more than two burstamplitudes. Two examples of such schemes are disclosed in U.S. Pat. No.6,122,133 and U.S. Pat. No. 5,781,361, which examples are incorporatedherein by reference. Such schemes would also benefit from the currentinvention.

FIGS. 4( e) and 4(f) show the results of subsequent steps of theservowriting process, which has produced a number of servo tracks. Afterthe first step in this process, each subsequent step writes one servoburst in a wedge and trims another. Every second step also writesdigital information, including the SAM and track number. Betweenservowriting steps, the servowriting head is stepped by one-half servotrack radially, either toward the inner diameter (ID) or outer diameter(OD) of the disk, depending on the radial direction used to write theservo information. A seek typically takes anywhere from one quarter toone half of the time it takes for the disk to make one revolution. Theprocess of writing the servo pattern for each servowriting steptypically takes one full revolution to write all of the wedges for thatstep. Using this algorithm, then, servowriting can take about 1.25–1.5revolutions per servowriting step. Since there are two servowritingsteps per servo-track in this example, and 1.5 servo tracks perdata-track, such a process requires 3 servowriting steps per data-track,or 3.75–4.5 revolutions per data-track. For purposes of subsequentdiscussion only, it will be assumed that the process takes 4 revolutionsper data-track.

A disk drive can have tens of thousands of data tracks. With 100,000data-tracks and a spin-speed of 5400 RPM (90 Hz), for example, theprocess would take 4,444 seconds, or about 75 minutes. If the process iscarried out on an expensive servowriter, this can add substantially tothe cost of the drive. Thus, drive manufacturers are motivated to useself-servowriting techniques to reduce or eliminate the time that adrive must spend on a servowriter.

One such technique uses a media-writer to write servo patterns on astack of disks. Each disk is then placed in a separate drive containingmultiple blank disks, such that the drive can use the patterned disk asa reference to re-write servo patterns on all of the other disk surfacesin the drive, as well as writing a servo pattern on the patternedsurface, if desired. The media-writer can be an expensive instrument,and it may still take a very long time to write a reference pattern onthe stack of disks. However, if a stack contains 10 blank disks, forexample, then the media-writer can write the reference pattern for 10drives in the time that it would have taken to servowrite a singledrive. This scheme is a member of a class of self-servowritingtechniques commonly known as “replication” self-servowriting.

A typical replication process, in which a drive servos on the referencepattern and writes final servo patterns on all surfaces, takes placewhile the drive is in a relatively inexpensive test-rack, connected toonly a power-supply. The extra time that it takes is therefore usuallyacceptable.

Another class of self-servowriting techniques is known as “propagation”self-servowriting. Schemes in this class differ from those in the“replication” class in the fact that the wedges written by the drive atone point in the process are later used as reference wedges for othertracks. These schemes are thus “self-propagating”. Typically, suchschemes require a R/W head that has a large radial offset between theread and write elements, so that the drive can servo with the readelement over previously-written servo wedges while the write element iswriting new servo wedges. In one such application, a servowriter is usedfor a short time to write a small “guide” pattern on a disk that isalready assembled in a drive. The drive then propagates the patternacross the disk. In this type of self-servowriting operation, previouslywritten tracks can later serve as reference tracks.

Many of the self-servowriting techniques, including those describedabove, require considerably more than four disk revolutions perdata-track written, as the drive must spend considerable time at thestart of each servowriting step determining the written-in runout of thecorresponding reference track, so that the servowriting head can beprevented from following that runout while writing the final servopattern. Techniques exist which allow tracks of servo information to bemade substantially circular, despite the fact that the referenceinformation is not perfectly circular.

The information used to remove written-in runout from the track can becalculated, in one approach, by examining a number of parameters over anumber of revolutions. These parameters can include wedge offsetreduction field (WORF) data values. WORF data can be obtained, forexample, by observing several revolutions of the position error signal(PES) and combining the PES with servo loop characteristics to estimatethe written-in runout, such as of the reference track. It is alsopossible to synchronously average the PES, and combine thesynchronously-averaged PES with servo loop characteristics to estimatethe written-in runout. Various measurements can be made, as are known inthe art, to characterize servo loop characteristics. Because the servotypically suffers both synchronous and non-synchronous runout, anymeasurement intended to determine the synchronous runout will beaffected by the non-synchronous runout. If many revolutions of PES dataare synchronously averaged, the effects of the non-synchronous runoutcan lessen, leaving substantially only synchronous runout. This allowsbetter determination of, and subsequent elimination of, the written-inrunout. Averaging many revolutions of PES data, however, can addsignificantly to the time required for determination of the written-inrunout. Process engineers may need to balance the cost and benefit ofadditional revolutions of PES data collection in determination of WORFvalues.

The computed written-in runout values for each servo wedge can bewritten into the servo wedges themselves for later use by the servo, orcan be kept in drive microcontroller memory for immediate use. During aself-servowriting operation, the drive may use the latter option bycalculating the written-in runout on a reference track and applying itto the servo by the use of a table in microcontroller memory. Additionalrevolutions of PES measurements for the reference track can be used toreduce the effects of non-synchronous, or repeatable, runout.

As previously described, techniques for determining and removingwritten-in runout of a track will hereinafter be referred to as WORFtechnology. If, for example, a drive spends 5 revolutions to determinethe written-in runout of each reference track before writing thecorresponding final wedges, that would add 15 revolutions to the writingtime of each data-track (5 extra revolutions per servowriting step,times 3 servowriting steps per data-track), bringing the total time perdata-track to 19 revolutions.

Even though the self-servowriting time may be as much as about fivetimes as long as the time necessary to servowrite a drive on aservowriter (19 revolutions/data-track, versus 4revolutions/data-track), self-servowriting is likely to be a lessexpensive alternative due to the expense of servowriters, as well as thefact that servowriting operations on a servowriter generally must beperformed in a clean-room environment. Also, as track-densities gethigher it becomes more difficult for an external device such as anactuator push-pin to control the position of the R/W heads accuratelyenough to produce a servo pattern with sufficiently small written-inrunout. The expense of servowriting also rises in proportion to thenumber of tracks on a drive.

FIGS. 4( a)–(f), described above, show an idealized servowriting processin which the radial placement of the writer is virtually perfect duringservowriting. In reality, the writer placement will not be perfect, evenif the written-in runout of the reference pattern is completely removed,due to non-synchronous positioning errors. There are several sources ofnon-synchronous runout, which is commonly referred to in the industry asNRRO, or Non-Repeatable Runout. If the servowriting head suffersnon-synchronous runout while writing servo wedges, that runout will bewritten into those wedges.

Such a result is illustrated in FIGS. 5 and 6. For the sake ofsimplicity, only A and B bursts are shown, leaving out the digitalinformation and other bursts. In FIG. 5, the servowriter head 402 in afirst servowriting step 400 writes an A-burst. In FIG. 6, the head 508in a second servowriting step is offset, or mis-placed, a distance fromits ideal position. For example, the ideal placement 502 of the top edgeof the writer, and therefore the ideal placement of the servo trackcenterline, is shown a distance from the actual placement 500 of the topedge of the writer 508. This separation is the written-in runout 506.

An algorithm such as a typical quadrature servo detection algorithm canbe used to determine the fractional track position of the R/W head bycomparing the amplitudes of two bursts that are 180 degrees out of phasewith one another, such as the A-burst and B-burst in FIG. 8. Becauseexisting servowriting processes involve trimming the A burst at the sametime the B burst is written, any mis-placement of the writer during thatrevolution will result in equal mis-placement of their common edges. Thewritten-in runout of that wedge, or the mis-placement of the center ofthe servo track, therefore will be equal to the mis-placement sufferedby the writer at the time of writing of the wedge. If the only runoutsuffered by the writer is NRRO (i.e., if the written-in runout of thereference wedges is completely eliminated by the use of WORFtechnology), then the Root-Mean-Square (RMS) written-in runout will beequal to the RMS NRRO suffered by the writer during the servowritingprocess.

FIG. 7 depicts a process in accordance with one embodiment of thepresent invention by which the RMS written-in runout can besubstantially reduced, at the cost of an extra revolution of the diskfor each servowriting step. For such a servowriting operation, thewriting of the B burst and the trimming of the A burst occur on separaterevolutions of the disk (i.e., on separate passes) with the A burst andthe B burst having a substantially common edge as an exactly common edgeis generally impractical. The mis-placement of the upper edge of the Bburst and the lower edge of the A burst, each of which are determined bythe mis-placement of the writer during the corresponding revolutions ofthe disk, are quasi-independent random variables. If the runout sufferedby the writer is indeed non-synchronous (NRRO), then the mis-placementsof those two burst edges should not be the same from one revolution tothe next. The mis-placement of the centerline of the servo track will beequal to the average of the misplacement of the upper edge of theB-burst and the lower edge of the A-burst.

For example, in the first servowriting step 600 of FIG. 7 the head 606writes an A-burst. In a first pass of the second servowriting step 602,the head 606 writes a B-burst. The head is displaced a first distance608 when writing the B-burst. In a second pass of the secondservowriting step 604, the head is displaced a different distance 610from the expected position when trimming the A-burst, leaving a smallerwritten-in runout 612 than would have occurred had the A-burst beentrimmed in a single pass of the second servowriting step 602 (whichwould have been approximately equal to the misplacement 608 on thatpass). The A-burst also could have been trimmed before writing theB-burst.

It is well known that the average of two un-correlated random variablesof RMS magnitude, r₀, as well as a mean value of zero, has an RMSmagnitude of:r ₀/√{overscore (2)}.Whether or not the two misplacements are truly un-correlated dependsupon the spectrum of the NRRO. Very low frequency NRRO components mayhave some correlation from revolution to revolution, but most NRROcomponents can be essentially un-correlated from one revolution to thenext. Thus, by spending one extra revolution for each servowriting step,the servowriting process can achieve about a 29% reduction in theresulting written-in runout.

This approach can be extended, as shown in FIG. 8. On a secondservowriting step 700, in which a B-burst is written, the head 714 has afirst mis-placement 706. At the cost of yet another revolution, thetrimming of the A burst can be done in two separate revolutions,trimming half of the burst in each revolution. On a second pass of thesecond servowriting step 702, half of the A-burst is trimmed with asecond mis-placement 708. On a third pass of the second servowritingstep 704, the other half of the A-burst is trimmed with a thirdmis-placement 710. The written-in runout 712 is then even smaller whenthree mis-placements are averaged. The track centerline being determinedby the un-correlated writer misplacement over three separate revolutionscan result in an additional reduction in written-in runout of about 13%.The concept can be further extended by trimming the A-burst inadditional passes, such as by trimming a third of the A-burst in each ofthree passes.

The written-in runout also can be reduced by writing the B-burst inmultiple passes, if the servowriting system is capable of writingmagnetic transitions with very high timing coherence from revolution torevolution (i.e., it is capable of lining up the transitions from twoseparate revolutions very accurately). For example, the B-burst can bewritten by writing a third of the burst in each of three separatepasses. The writing passes and trimming passes can involve doing all thewriting then all the trimming, all the trimming then all the writing ofnew bursts, or alternating trimming and writing. If the separateportions of the B-burst are not circumferentially lined up veryaccurately, though, a burst-amplitude demodulation scheme could give aninaccurate measurement of the overall amplitude of the burst, and canactually increase the written-in runout. If the B-burst were to be splitinto two separate B-bursts, with each being demodulated separately andthe amplitudes being averaged, then this would not be a problem. Thiscould require a servo detection system that can accommodate more bursts,as well as additional overhead on the disk for the necessary spacebetween the sub-bursts. If the techniques shown in FIG. 7 or 8 are used,there is no need for extremely high coherency in servowriting.

If the embodiment depicted in FIG. 7 is applied to a standardservowriting operation done on a servowriter, one extra revolution perservowriting steps is required. With three servowriting steps perdata-track, this will nearly double the time spent on a servowriter,jumping from four revolutions per data-track to seven. This is likely tobe an unacceptable cost for most applications. If the same innovation isapplied to a self-servowriting operation as described above, however,the relative increase in servowriting time is much smaller. Since theself-servowriting time is already nineteen revolutions per data-track,an additional three revolutions adds only about 16% to theself-servowriting time, which is much less expensive than servowritertime.

If the procedure depicted in FIG. 8 is applied to a servowriter ormedia-writer operation, the time will be 150% greater than the standardservowriter time, taking into consideration the original fourrevolutions per data track plus three additional revolutions for each ofthe two separate erases. In the self-servowriting case, the additionalsix revolutions per data-track add only about 32% to the time. Whilesuch a technique may add an unacceptable cost to a servowriter ormedia-writer process, the process may add an acceptable cost to aself-servowriting process.

FIG. 9 shows a method that can be used in accordance with one embodimentof the present invention. In this exemplary method, the writing of anA-burst/B-burst boundary is described. It should be understood that themethod can be used with any set of bursts in any order. In this method,an A-burst is written on a first revolution of a disk 800 upon which aservo pattern is to be written. At least a portion of the A-burst istrimmed on a subsequent rotation of the disk 802. If the A-burst is notcompletely trimmed 804, step 802 can be repeated until the entireA-burst is trimmed. At least a portion of the B-burst for the givenburst boundary can be written on a subsequent revolution 806. If theB-burst is not completely written 808, step 806 can be repeated untilthe entire B-burst is written. The writing and trimming of the burstboundary is then complete 810. It should be understood, however, thatsteps 802–804 can be done in any reasonable order. For example, steps806 and 808 for writing the B-burst can occur before steps 802 and 804for trimming the A-burst. It is also possible to intersperse the writingof the B-burst with the trimming of the A-burst, such that you couldwrite the A-burst, write at least a portion of the B-burst, trim atleast a portion of the A-burst, then write another portion of theB-burst.

Additional Passes at Less than all Boundaries

In order to take advantage of the reduction in written-in runout withoutadding unacceptable cost, other embodiments in accordance with thepresent invention take advantage of additional servo passes only forspecific burst boundaries or for specific servowriting steps. Such anembodiment can be a useful balance for certain applications where animprovement in head position control during write operations is desired,but the amount of extra time needed to take additional revolution(s) foreach track is determined to be unacceptable or undesirable. For example,in FIG. 10 a series of A/B boundaries is used to define a centerline 900for a track of data. The centerline 900 ideally passes through, adjacentto, or near an A/B boundary in each servo wedge on the disk asoriginally designed, although the subsequent removal of written inrunout may cause a centerline to pass near other boundaries. Anothersuch track centerline 906 is defined by a series of C/D boundaries inservo wedges about a disk. Other boundaries can define lines 902, 904that may be used for purposes such as reading and/or positioning, but donot define track centerlines.

Instead of writing and trimming on separate revolutions of the disk foreach burst boundary line, such as described above, it is possible toselect certain boundary lines with which to utilize extra revolutions.For the non-selected boundary lines, it may be decided that the positionerror signal values are not critical enough to warrant the time/cost itwould take to use additional servo passes to write and/or trim thebursts. For example, in FIG. 10 it might be decided that extrarevolutions will be used for track centerlines 900, 906 because it isvery important to write in approximately the desired location in orderto avoid damaging data. It might be acceptable, however, to have greateruncertainty in read lines 902, 904 that are used for reading operations,as additional reads can be done without damaging data. If an incorrectPES results in an improper read operation, a number of things can bedone as are known to those of ordinary skill in the art to attempt toproperly read the data, such as moving the head slightly off trackcenter while reading data during subsequent revolutions.

In the pattern of FIG. 10, which is a three-step per track pattern,(sometimes also referred to in the trade as a “three-pass per track”pattern) using such a method would mean that extra revolutions wouldonly be used for every third servowriting step, such as servo tracks 900and 906 of servo tracks 900,902,904,906. In other patterns, the numberof tracks utilizing extra passes could be increased or decreasedaccordingly. The use of extra passes for less than all servo tracks canbe done for either servo writing or self-servowriting. In the case ofservowriting, for a three pass per track pattern there are about fourrevolutions per track, as there are three full steps and about a thirdof a revolution per seek. Adding an additional pass to separately trim aburst at every servowriting step, for example, would increase the numberof revolutions from four to seven, for about a 75% increase in passesand therefore servo time. This may be unacceptable on a servowriterwhere increasing write time can be very expensive. If, instead, theservowriter only uses an extra revolution for every data track center,or every third servowriting step in a three-pass-per-track servopattern, then the increase in servo time is approximately 25%. This maybe an acceptable trade-off to improve the written-in runout.

With a self-servowriting process, the additional overhead due toadditional revolutions can be reduced from the 16% increase when takingan additional pass for each servowriting step. As discussed above, astandard self-servowriting process can include about nineteenrevolutions per data-track, so adding one additional revolution per datatrack only adds about 5% to the self-servowriting time, which is muchless expensive than servowriter time.

Other embodiments provide for the use of additional passes at less thanall boundaries using other determinable and/or variable criteria. Forexample, it can be desirable to increase the number of revolutions orservowriting steps that are used to write bursts defining trackboundaries as a write element moves from writing servo information nearthe inner diameter (ID) of a disc towards the outer diameter (OD) of thedisc. In certain systems, the amount of synchronous and/ornon-synchronous runout can increase as a write head approaches the outeredge of the disc. Reasons for this can include, for example, an increasein vibration of the disc toward the outer edge or turbulence effects dueto the increase in tangential velocity near the OD. Therefore, it can bedesirable to increase the number of revolutions or servowriting stepsused to define track boundaries or centerlines, or at least write trackcenterlines, from the ID to the OD. In some systems, data zones can beused to save space on the disk and allow a greater amount of data to bestored near the OD than near the ID. In such a system, it is possible touse a different number of revolutions to write track boundaries andtrack centers in each data zone. For example, in a system with threedata zones, the zone nearest the ID might use 2 revolutions to write ortrim a burst for a data track centerline, writing or trimming one halfof the burst on each revolution. An intermediate zone might use 3revolutions each writing 1/3, while the zone nearest the outer diametermight use 4 revolutions. For servo track boundaries, for example, thezones might use 1, 2, and 3, respectively. Alternatively, the servotrack boundaries might each use 1, or the two data zones nearest the IDmight use 1, and the data zone nearest the OD might use 2. It is alsopossible to adjust the number of revolutions dynamically by monitoringthe precision of the process as the servo data is being written to thedrive.

Different Numbers of WORF Revolutions at Different Boundaries

Just as the number of burst writing passes used per servowriting stepcan vary from one burst boundary to another, the number of revolutionsused to collect PES information to determine the written-in runout ofthe reference pattern can also vary similarly. If, for example, fiverevolutions of PES data collection would normally be used to determinethe written-in runout at each burst boundary, a drive can collect ninerevolutions of data before writing a burst boundary that determines thewrite track center of the final pattern, but may use only threerevolutions to collect data before writing other boundaries. Thisexemplary approach could still consume fifteen revolutions perservowritten data-track, three revolutions for each of two boundariesplus nine revolutions for the third. The removal of written-in runout ofthe reference pattern, and hence the quality of the final patternboundaries, could be improved at the critical boundaries and degraded atthe non-critical boundaries, with no change in the overallself-servowriting time. For another example, if five revolutions of PESdata collection were used at the non-critical boundaries and tenrevolutions were used at the critical boundary, the written-in runout atthe critical boundary could be improved without any degradation of thenon-critical boundaries, at the cost of an additional five revolutionsper servowritten data-track.

Other embodiments provide for the use of different numbers of WORFrevolutions at less than all boundaries using other determinable and/orvariable criteria. For example, it can be desirable to increase thenumber of WORF revolutions taken when writing bursts defining trackboundaries as a write element moves from writing servo information nearthe inner diameter (ID) of a disc towards the outer diameter (OD) of thedisc. As discussed above, the amount of synchronous and/ornon-synchronous runout can increase as a write head approaches the outeredge of the disc. Therefore, it can be desirable to increase the numberof WORF revolutions used to define track boundaries or centerlines, orat least write track centerlines, from the ID to the OD. In somesystems, data zones can be used to save space on the disk and allow agreater amount of data to be stored near the OD than near the ID. Insuch a system, it is possible to use a different number of WORFrevolutions to write track boundaries and track centers in each datazone. For example, in a system with three data zones, the zone nearestthe ID might use 5 WORF revolutions for a data track centerline. Anintermediate zone might use 7 WORF revolutions, while the zone nearestthe outer diameter might use 9 or 10 revolutions. For servo trackboundaries, for example, the zones might use 5, 6, and 7, respectively.Alternatively, the servo track boundaries might each use 5, or the twodata zones nearest the ID might use 5, and the data zone nearest the ODmight use 6. It is also possible to adjust the number of WORFrevolutions dynamically by monitoring the precision of the process asthe servo data is being written to the drive.

Additional Servo Pattern Component(s)

In other embodiments, it is possible to extend a servo pattern such asthose described above, and use that extended pattern with multi-passservowriting. As mentioned above, it is possible to write bursts inmultiple passes by writing at least a portion of a burst during eachpass. A problem exists, however, in that it can be difficult tocoherently write separate portions of the same burst. If there is someincoherence between the portions, demodulating the burst by a processsuch as taking a discrete Fourier transform (DFT) of the whole burst canreturn a result of approximately zero if the two portions aresufficiently out of phase. Writing the bursts in separate passes andreading them in the same pass may then result in adding as much RRO asis being saved, or even possibly degrading the RRO.

One way to avoid such a coherence problem is to write separate bursts,instead of writing separate portions of the same burst. In one approach,a second burst can be written for each burst on every track. This isshown, for example, in FIGS. 11 and 12. In FIG. 11, there is anadditional burst written for each burst in the pattern. Track centerline1002 is then defined by not only an A/B boundary, but is defined by anM′/BB′ boundary. The AA′/BB′ boundary includes the lower edge of anA-burst and a companion burst that shall be referred to herein as anA′-burst, and the pair of edges will be referred to as AA′. The B-burstalso has a companion burst, referred to as a B′-burst, and the pair isreferred to herein as BB′. A coherence problem can still exist in such apattern if the A-burst and A′-burst are demodulated as a single burst.However, if they are demodulated separately, such as by applying aseparate DFT demodulation to each burst individually, such problems canbe avoided.

An aspect to such an exemplary demodulation scheme that might beundesirable, however, is the fact that such a scheme can require eightseparate servo bursts, and eight separate burst demodulations for eachservo wedge. This may be undesirable for a number of reasons. First, notall drives may have the chips or circuitry necessary to handle eightbursts. More important, however, will be the fact that manufacturerswould rather not give up the disk capacity necessary to add theadditional four burst regions to have an additional burst for each burstin a servo wedge.

One way to address such a concern is shown in FIG. 13. The exemplarypattern shown in FIG. 13 only uses six servo burst regions, or two morethan are used in a nominal four-burst pattern. This pattern takesadvantage of the fact that it is relatively safe to trim a burst inmultiple portions. Taking this advantage into consideration, it ispossible to have four edges used for each boundary but only write oneadditional burst. For example, track centerline 1202 is defined by anA/BB′ burst. The addition of the B′-burst means that two bursts arewritten on separate passes, with the upper edge of each burst capable ofbeing used for the PES without coherence concerns. It is still possible,however, to trim the A-burst in at least two passes, such that theoverall written-in runout is reduced. The two edge portions of thetrimmed A-burst, as well as the edges of the B and B′ bursts, stillprovide four edges for the PES, although the PES noise from the A-burstedge portions may be greater than if there were two complete burst edgesused.

In order to save servo time, as well as to address interferenceconcerns, it is possible to simply add a complimentary servo burst foreach data track centerline, instead of each servowriting step. As shownin the example of FIG. 13, a B′ burst is written for the boundarydefining track centerline 1202, and a C′ burst is written for theboundary defining track centerline 1208, but there is not an additionalburst for the boundaries defining read lines 1204 and 1206. This allowsthe additional bursts to be written using only two additional burstregions 1210, for a total of six burst regions. In the Figure, forexample, the B′-bursts and A′-bursts are written in the same burstregion, while the C′-bursts and D′-bursts are written in another burstregion.

In certain embodiments, it may be possible to write all the additionalbursts in the same burst region. This may not be desirable for today'sdrives, however, since that would allow only one-third of a data trackbetween the bursts, each of which is about two-thirds of a data track inwidth. Since the width of a read/write element can be about as wide as adata track, any misplacement of the head and/or servo data could resultin an incorrect value of PES being determined, as there would be acontribution due to the detection of a portion of a servo burst for anadjacent track. Further, if the reader is any wider than a third of atrack, the amplitude of the signal read from the additional bursts willnever go to zero, but will always read some amplitude. This canintroduce some uncertainty into the measured PES values.

In other embodiments, a pattern can be used that differs from ID to ODfor reasons such as those discussed above. For example, if a disc hasthree data zones, the data zone nearest the ID might not use anyadditional bursts to define a track centerline or track boundary. Anintermediate zone might use one extra burst to define a track boundaryor centerline, or just a data track centerline. The data zone nearestthe OD might use two extra servo bursts to define a track boundary orcenterline, or just a data track centerline.

Although embodiments described herein refer generally to systems havinga read/write head that can be used to write bursts on rotating magneticmedia, similar advantages can be obtained with other such data storagesystems or devices. For example, a laser writing information to anoptical media can take advantage of additional passes when writingposition information. Any media, or at least any rotating media, uponwhich information is written, placed, or stored, may be able to takeadvantage of embodiments of the present invention.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many modifications andvariations will be apparent to one of ordinary skill in the relevantarts. The embodiments were chosen and described in order to best explainthe principles of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications that are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims and their equivalence.

1. A method for writing position information to a rotatable medium,comprising: selecting a pattern comprising a plurality of concentrictracks, wherein each concentric track is defined by a plurality of burstboundaries positioned circumferentially about a rotatable storagemedium, the rotatable storage medium having an inner diameter and anouter diameter; writing a first servo burst and a second servo burst foreach burst boundary to the rotatable storage medium, wherein the firstservo burst and second servo burst are written on separate passes of awrite element over the rotatable storage medium, and wherein the firstservo burst and second servo burst each have an edge that can be used todetermine the position of the write element during a subsequent passover those servo bursts; and trimming the first servo burst for eachburst boundary, wherein a subset of the plurality of concentric tracksis selected wherein the burst boundaries defining that subset use aseparate pass of the write element to trim the first servo burst, thesubset being selected based upon the proximity of each concentric trackto at least one of the inner diameter and outer diameter.
 2. A methodaccording to claim 1, further comprising: using the trimmed edge of thefirst servo burst and an adjacent edge of the second servo burst todetermine the position of the write element.
 3. A method according toclaim 1, wherein: trimming the first servo burst includes trimming thefirst servo burst to have a width approximately equal to the width of atrack of servo data.
 4. A method according to claim 1, wherein: thefirst and second servo bursts are contained in a servo wedge on therotatable storage medium.
 5. A method according to claim 1, wherein: thetrimmed edge of the first servo burst and an adjacent edge of the secondservo burst define the position of a centerline of a data track on therotatable storage medium.
 6. A method according to claim 5, wherein thesubset further includes each first and second servo burst that defines adata track centerline.
 7. A method according to claim 1, wherein:writing the second servo burst occurs before trimming the first servoburst.
 8. A method according to claim 1, wherein: the subset is selectedfurther based on the amount of runout.
 9. A method for writing positioninformation to a rotatable storage medium having servo tracks and datatracks written thereon, comprising: writing a plurality of servo tracksto a rotatable storage medium having an inner diameter and an outerdiameter, wherein the position of each servo track is defined by an edgeof a first servo burst and a complimentary edge of a second burst, andwherein the first servo burst is written in a first revolution of therotatable storage medium, and the first burst is trimmed on a secondrevolution, the second servo burst being written on one of the secondrevolution and a subsequent revolution depending on the proximity to theouter diameter; and writing a plurality of data tracks to a rotatablestorage medium, wherein the position of each data track is defined by anedge of a third servo burst and a complimentary edge of a fourth servoburst, and wherein the third servo burst is written in a thirdrevolution of the rotatable storage medium, the third servo burst istrimmed in a fourth revolution, and the fourth servo burst is written onone of the fourth revolution and a subsequent revolution, depending uponthe proximity to the outer diameter.
 10. A method according to claim 9,wherein: data tracks having a fourth servo burst written on a fourthrevolution are closer to the inner diameter than data tracks having afourth servo burst written on a subsequent revolution.
 11. A method forwriting position information to a rotating medium, comprising: writingat least a portion of a first burst pattern during a first pass of awrite element over a rotating medium; trimming at least a portion of afirst burst pattern during a second pass of the write element; writingat least a portion of a second burst pattern during one of the secondpass and a subsequent pass of the write element, depending on theproximity to an outer diameter of the rotating medium.
 12. A method formanufacturing a hard disk drive, comprising: providing means forselecting a pattern comprising a plurality of concentric tracks, whereineach concentric track is defined by a plurality of burst boundariespositioned circumferentially about a rotatable storage medium, therotatable storage medium having an inner diameter and an outer diameter;providing means for writing a first servo burst and a second servo burstfor each burst boundary to the rotatable storage medium, wherein thefirst servo burst and second servo burst are written on separate passesof a write element over the rotatable storage medium, and wherein thefirst servo burst and second servo burst each have an edge that can beused to determine the position of the write element during a subsequentpass over those servo bursts; and providing means for trimming the firstservo burst for each burst boundary, wherein a subset of the pluralityof concentric tracks is selected wherein the burst boundaries definingthat subset use a separate pass of the write element to trim the firstservo burst, the subset being selected based upon the proximity of eachconcentric track to at least one of the inner diameter and outerdiameter.